1. Field of the Invention
The present invention relates to a composite material and to a method for producing it. Specifically, the present invention relates to a composite material produced by forming graft polymers nonuniformly on a substrate and also to a method for producing this composite material. More specifically, the present invention relates to a biodegradable composite material capable of exhibiting an excellent biodegradability together with other functions and also to a method for producing this biodegradable composite material.
2. Description of the Related Art
Because of the disposal problems of discarded goods such as synthetic plastics, a biodegradable resin, which is degraded by the action of microorganisms in soil, is drawing attention. Presently, known biodegradable resins include aliphatic polyesters and the like which are prepared by a chemical synthesis or a biosynthesis, resins utilizing naturally occurring polymers such as starch, cellulose, and the like, and chemically synthesized polymers such as polyvinyl alcohol, polyether, and the like. These materials are described in, e.g., Yoshiharu Doi, xe2x80x9cBiodegradable Polymeric Materialsxe2x80x9d (Kogyo Chosakai Publishing Co., Ltd.).
Despite remarkable progress in biodegradable resin technologies, the use of biodegradable resins centers on wrappers and containers which are discarded after a single use, and the amount of use the biodegradable resins is also restricted by their high cost. Henceforth, biodegradable resins will expand their field of application as a highly functional material in addition to the wrappers and containers now in use. However, conventional biodegradable resins are mostly developed from the standpoint of degradability and, hence, the restrictions imposed on designing retard the development of biodegradable resins as functional materials.
Meanwhile, materials, which effectively utilize naturally occurring polymeric materials, are highlighted because of problems such as the depletion of petroleum resources and the release of CO2 into atmosphere and, hence, one of the contemplated applications of naturally occurring resins is their use as a biodegradable resin. However, the attempt to impart functions to the naturally occurring polymeric materials is associated with problems. For example, one of the problems is that chemical modification impairs the excellent biodegradability inherent in the naturally occurring polymeric materials. Another problem is that it is difficult to obtain materials having a high level of functions comparable to those of synthetic polymers by a mere chemical modification of the substituent groups of the naturally occurring polymeric materials. As an embodiment of a naturally occurring polymeric material rendered more highly functional, a material which comprises a naturally occurring polymeric material having grafted thereto other polymeric material has been developed (J. Fibers and Textiles Soc. Japan, 102, Vol. 47, No.2, 1991). However, little is reported of the studies about the biodegradability of such a material.
A variety of graft polymerization methods are known as an effective means for modifying a material or a material surface. Examples of these methods include (1) a method wherein active sites are created either inside a material or on the surface of the material by utilizing the reaction between either radioactive rays, a redox initiator, or an ion-based initiator and the material or by utilizing the chain transfer of a radical initiator and thereafter a polymerizable unsaturated compound (hereinafter referred to as xe2x80x9ca polymerizable monomerxe2x80x9d on occasion) is polymerized by the active sites (described in, e.g., Adv. Polym. Sci., Vol. 4, p.111, 1965), (2) a method wherein polymerizable unsaturated bonds are introduced into a material and a polymerizable monomer is polymerized by the unsaturated bonds (described in, e.g., J. Polymer Sci., Part-A, 3, p.1031, 1965 and Japanese Patent Application Publication (JP-B) No. 5-5,845), and (3) a method wherein a structure which acts as an initiator is introduced into a material and a polymerizable monomer is polymerized by the structure (described in, e.g., Tappi, March, 56, p.97, 1973 and Japanese Patent Application Laid-Open (JP-A) No. 6-287,243).
Among these methods, method (1) is preferably employed, and a system utilizing an initiator in particular is widely employed.
As a graft polymerization initiator (hereinafter referred to simply as xe2x80x9can in initiatorxe2x80x9d on occasion), a redox initiator is industrially employed, and conventionally known examples of the redox-based initiator include ammonium cerium(IV) nitrate, Fenton""s reagent (an H2O2/Fe system and the like), and a manganese-based system. According to graft polymerization using these initiators, the polymerization is generally performed in water or in a solvent system mixed with water. Details of these graft polymerization technologies illustrative of an example of a graft polymerization onto a hydrophilic polymer are described in, e.g., Fusayoshi Masuda, xe2x80x9cHighly Water-absorbing Polymer, Polymeric New Materials, One Point-4xe2x80x9d, Polymer Soc. Japan. Ed. and xe2x80x9cJ. Appl. Polymer Sci., 19, p.1257, 1975xe2x80x9d.
According to conventional graft polymerization, the amount of the polymer grafted (generally referred to as a grafting ratio) onto a target material or the surface of a substrate to be grafted is controlled by the selection of an initiator, the type or structure of the initiation site, the concentration of the initiator, the concentration of a polymerizable monomer, and a solvent.
However, in a system according to a conventional method for graft polymerization, in particular a system using a redox initiator, it is difficult to control the molecular weight of the graft polymer and the grafting ratio, and it is also difficult to control the high-order structures, such as the morphology and the areal density, of the graft polymer which is formed on a substrate. That is, although it is possible to change the number of polymerizable monomers, which become the graft polymers, bonded to a unit area on the substrate surface by changing, for example, the concentration of the initiator, it has been impossible to make nonuniform the bonding density of the graft polymers on the surface of a substrate, for example, to obtain a surface structure comprising a region (domain) which has a plurality of graft polymers bonded and a non-grafted domain. Despite an attempt to obtain the above-described high-order structure by a method comprising partially masking the surface of a substrate and then performing a graft polymerization onto only the non-masked region (JP-B No. 62-7,931), this method is not practical because of limitations such as complexity of process and, in addition, this method cannot be applied to polyhedral substrates such as fibers and particles.
The high-order structural modification of the substrate surface as described above is useful as a means of modifying the surface physical properties, such as steric space, electrical potential, and free energy, in various applications, but conventional methods for this purpose are not satisfactory because the degree of freedom in controlling the high-order structure is low.
For the purpose of obtaining a so-called functional surface combining a plurality of functions on the substrate surface, a method hitherto employed consists of utilizing as a copolymer a graft polymer itself which is grafted onto a substrate. The problem of this method is that the modification of surface properties achieved by this method is only of microscopic order, and, therefore, the modification of surface properties by this method cannot provide the functions to match those which can only be achieved by some degree of macroscopic assembly of graft polymers.
A still further problem of the conventional method for graft polymerization has been that graft polymerization performed in an aqueous solvent produces a large amount of a polymer product not grafted onto the target material (this polymer is hereinafter referred to as xe2x80x9ca homopolymerxe2x80x9d), which leads to the reduction in reaction efficiency and an enormous amount of work required in the purification of the reaction product. Although the formation of a homopolymer is inhibited by a technology (A.C.S., Symp. Ser., No. 187, p.45, 1982) comprising the steps of reacting an initiator with a material or a substrate in an aqueous solvent, exchanging the aqueous solvent with a non-aqueous solvent, and performing graft polymerization, this technology is complicated and not suitable to industrial use because the solvent exchange must be carried out under an inert gas.
A first object of the present invention is to provide a composite material comprising a substrate, having a three-dimensional structure, which is composed of a polymeric material and has a polymeric material, different from the polymeric material of the substrate, chemically bonded to at least the surface of the substrate, wherein the fine structure on the surface of the substrate having a three-dimensional structure is nonuniform such that the fine structure has a nonbonded domain where the polymeric compound is not bonded.
A second object of the present invention is to provide a biodegradable composite material comprising a substrate having a three-dimensional structure which is composed of a biodegradable polymeric material and has a polymeric material, different from the polymeric material of the substrate, chemically bonded by means of a graft polymerization to at least the surface of the substrate, wherein the fine structure of the surface of the substrate having a three-dimensional structure is nonuniform such that the fine structure has a nonbonded domain where the polymeric compound is not bonded.
A third object of the present invention is to provide a method for graft polymerization, said method having a high degree of freedom in controlling the amount, density, grafting ratio and the like of the graft polymers to be formed on a substrate having a three-dimensional structure, and in particular said method being able of controlling at will the surface structure comprising the graft polymers on the surface of substrate, for example, a high-order structure, such as a sea/island structure of (a mottled structure), composed of domains made up of an assembly of regions where the graft polymers are bonded along with non-grafted regions.
A fourth object of the present invention is to provide a method for graft polymerization, said method enabling even a non-aqueous, polymerizable monomer, which cannot be polymerized by a polymerization method in an aqueous solvent, to be easily graft-polymerized.
A fifth object of the present invention is to provide a method for graft polymerization, said method being capable of easily inhibiting the formation of a homopolymer in a polymerization process.
In the present invention, for use in biodegradable applications, preferred examples of the substrate having a three-dimensional structure, to the surface of which a polymerizable monomer is chemically bonded by graft polymerization, include biodegradable polymeric materials such as polysaccharide-based polymers, polypeptide-based polymers, polyester-based polymers, polyvinyl-based polymers and mixtures thereof.
The composite material of the present invention comprises a substrate, having three-dimensional structure, having, over substantially the entire surface thereof, regions where a polymerizable unsaturated compound is graft-bonded and regions where the polymerizable unsaturated compound is not graft-bonded.
The composite material of the present invention is prepared by a process comprising reacting the surface of a substrate having a three-dimensional structure composed of the polymeric material with droplets of a solution containing a graft polymerization initiator in a suspension or an emulsion comprising the solution containing a graft polymerization initiator and a solvent immiscible with the solution containing the graft polymerization initiator so as to form polymerization initiating regions on the surface of the substrate and then graft-polymerizing a polymerizable unsaturated compound to the polymerization initiating regions formed on the surface of the substrate. The fine structure on the surface of the substrate having a three-dimensional structure is nonuniform so that the fine structure has a nonbonded domain where the different type of polymeric material is not graft-polymerized.
The method for graft polymerization of the present invention comprises the step of reacting the surface of a substrate having a three-dimensional structure composed of the polymeric material with droplets of a solution containing a graft polymerization initiator in a suspension or an emulsion comprising the solution containing a graft polymerization initiator and a solvent immiscible with the solution containing the graft polymerization initiator so as to form polymerization initiating regions on the surface of the substrate and the step of graft-polymerizing a polymerizable unsaturated compound to the polymerization initiating regions formed on the surface of the substrate.
The method for graft polymerization of the present invention comprises a first graft polymerization step comprising the step of reacting the surface of a substrate composed of the polymeric material having a three-dimensional structure with droplets of a solution containing a graft polymerization initiator in a suspension, or an emulsion comprising the solution containing a graft polymerization initiator and a solvent immiscible with the solution containing the graft polymerization initiator, so as to form polymerization initiating regions on the surface of the substrate, and the step of graft-polymerizing the first polymerizable unsaturated compound to the polymerization initiating regions formed on the surface of the substrate and a second graft polymerization step of graft-polymerizing a second polymerizable unsaturated compound which is different from the first polymerizable unsaturated compound to the surface where the first graft polymerization does not take place.
The composite material of the present invention comprises the surface of a substrate composed of a polymeric material to which a polymeric compound of a different kind from the polymeric material chemically is bonded by graft polymerization, wherein the bonded state of the polymeric compound is nonuniform or partly present on the surface so that a domain (region) where the polymeric compound is not bonded is present. A preferred example of the surface structure is shown in FIG. 5A, and a cross section of the surface structure is shown in FIG. 5B. FIG. 5A illustrates a state of island-like domains where a plurality of polymeric compounds 2 of a different kind are present on the surface of a substrate 1. In FIGS. 5A and 5B, a numeral 3 indicates a domain where a different type of polymeric compounds are bonded, whereas a numeral 4 indicates nonbonded domain where the different type polymeric compounds are not bonded.
The nonbonded domain, which is present on the surface of the substrate of the composite material of the present invention and in which the polymeric compounds are not bonded, can take the form of an island or a continuous phase.
The substrate, which is used in the biodegradable material of the present invention, is preferably of a biodegradable material. In order to impart various functions to a biodegradable composite material, which is composed of the biodegradable material and a non-biodegradable material, without impairing the biodegradability, it is preferable that the nonbonded domain have at least a size which allows the formation of a true circle having a diameter of 5 nm or greater. This is because an enzyme molecule, which degrades the polymeric material of a substrate, and the steric effect of a composite material each play a role in the biodegradation of the polymeric material of the substrate.
According to the method for graft polymerization of the present invention, polymerization initiating sites can be selectively formed on a reacting surface by a process comprising dispersing an aqueous solution containing a graft polymerization initiator (this solution is hereinafter referred to as xe2x80x9can initiator solutionxe2x80x9d on occasion) in a state of a suspension or an emulsion in an organic solvent and then reacting the initiator solution with the surface of a substrate. Accordingly, it is possible to form graft polymers in a sea/island by selectively graft-polymerizing a polymerizable monomer to the domains having the polymerization initiating sites. That is, it is possible to form graft polymers so that a non-grafted region remains on the surface.
The method for forming a sea/island structure on the surface of a substrate according to the method of the present invention can find applications in, for example, controlling the electrostatic charge of electrophotographic materials such as toner and carriers, medical materials having a high compatibility with blood, and column materials for use in affinity chromatography and the like.
Further, if graft polymers of two or more types are formed on the surface of a substrate according to the method of the present invention, it is possible to obtain a surface having a plurality of functions derived from each type of graft polymer. The composite material thus obtained can be used in the aforementioned electrophotography, medical materials, chromatography, and the like.